1. Field of the Invention
The present invention relates to a method for producing a dimerized aldehyde which comprises subjecting an aldehyde to a condensation reaction and a dehydration reaction in the presence of a basic catalyst. Particularly, it relates to a method for producing a dimerized aldehyde at a high selectivity by suppressing formation of undesirable high boiling point compounds such as a trimer of aldehyde in the condensation reaction and the dehydration reaction of an aldehyde.
2. Discussion of Background
Heretofore, a method for producing a dimerized aldehyde has been known in which an aldehyde is subjected to a condensation reaction (also known as "aldol condensation" or "aldolization") and a dehydration reaction using a basic compound such as an aqueous alkaline solution as a catalyst. For example, when n-butyraldehyde (hereinafter referred to as NBD) is subjected to a condensation and a dehydration reaction, 2-ethylhexenal (hereinafter referred to as EPA) is obtainable. However, it is known that in the conventional condensation and dehydration reaction method, the dimerized aldehyde (a kind of dimers) as the desired product is likely to further react to form high boiling point compounds such as a trimer, a tetramer, etc. Consequently, the yield of the dimerized aldehyde tended to be low. To solve such a problem, the following various improvements have heretofore been proposed.
For example, Japanese Examined Patent Publication No. 24787/1964 discloses a method wherein NBD solution and an aqueous alkaline solution are counter-currently contacted in a column packed with a packing material or perforated plates, and vibration is given to this column to disperse NBD solution and thereby to prepare EPA.
Japanese Examined Patent Publication No. 43810/1977 discloses a method wherein NBD solution and an aqueous alkaline solution are reacted at a temperature of from 120.degree. to 130.degree. C. under a pressure of from 4 to 5 kg/cm.sup.2 G using two reactors i.e. a stirred tank reactor and a tubular reactor.
Further, French Patent 2,058,532 discloses a method wherein acetaldehyde is subjected to aldolization with a dilute sodium hydroxide aqueous solution by means of a perforated plate tower. It is disclosed that in order to carry out the reaction at a desired selectivity, the aldolization is stopped at a desired stage by adding acetic acid to the bottom of the column to neutralize the alkali catalyst. In this method, if it is attempted to conduct a dehydration reaction at the same time as the aldolization reaction of acetaldehyde by an alkali catalyst, enolate ions tend to form due to transfer of hydrogen at the .gamma.-position of croton aldehyde as a dimer, and the enolate ions are likely to repeat condensation to finally form a polymer. To prevent such a possibility, a kind of a reaction terminator, such as acetic acid, is introduced into the reaction system.
On the other hand, in an industrial operation of such a condensation and dehydration reaction, it is common that after separating from the reaction solution an oil layer containing a product such as EPA by such a method as oil-water separation, an aqueous layer being an aqueous alkaline solution, is recycled to the reactor for reuse. However, due to water formed by the dehydration reaction, the concentration of the alkali catalyst in the aqueous solution decreases. Further, as the volume of the aqueous alkaline solution increases, it is necessary to purge a part of the recycling aqueous alkaline solution depending upon the formed water. Accordingly, it has been necessary to supplement a basic catalyst afresh to the reactor to compensate the purged portion.
Further, the above-mentioned purged solution contains water polluting substances such as sodium hydroxide catalyst and sodium butyrate formed by a Cannizzaro reaction which partially takes place in the reactor. Accordingly, it is necessary to conduct treatment such as neutralization treatment to prevent pollution before discharging it as waste water, and a substantial investment for such an installation has been required.
However, by the method disclosed in Japanese Examined Patent Publication No. 24787/1964, the yield of formed EPA is at a level of 94% at best. Also in the method disclosed in Japanese Examined Patent Publication No. 43810/1977, large amounts of aldol products and high boiling point compounds will form, and unreacted NBD will be substantial at a level of a few %. Thus, in the prior art, a dimerized aldehyde such as EPA has not been obtained in satisfactory yield.
On the other hand, to solve such a problem, there has been proposed a method wherein high boiling point compounds formed as by-products, are decomposed into a feed aldehyde and an unsaturated aldehyde for recovery (Japanese Unexamined Patent Publications No. 24952/1964 and No. 17907/1964), or a method wherein before the aldol condensation product is introduced to the hydrogenation reaction step, high boiling compounds are separated therefrom in an evaporator, and the separated high boiling point compounds are recycled to the condensation reaction step, so as to improve the yield of the condensation step (Japanese Unexamined Patent Publication No. 41309/1976). However, if such a process is to be adopted on an industrial scale, the process steps will be complex, and a decomposition apparatus or a separating apparatus for high boiling point compounds will further be required. Accordingly, the installation cost will increase, such being economically disadvantageous.
Further, in the method disclosed in French Patent 2,058,532, acetic acid is added, whereby it is practically impossible to repeatedly reuse the alkaline catalyst. Therefore, such a method is commercially disadvantageous.
On the other hand, some proposals have been made to solve the water pollution problem.
For example, in Japanese Unexamined Patent Publication No. 28109/1978 by the present inventors, the reaction solution is subjected to oil-water separation, whereupon at least a part of the obtained aqueous phase is distilled, so that water corresponding to the amount of water formed by dehydration, is distilled off and discharged in the form which is free from a water polluting substance.
This method is an advanced method from the viewpoint of the environmental protection. However, in addition to the reactor, a distillation equipment is required, and thus a substantial investment for the installation is required. For this reason, this method is not satisfactory for an industrial application.
Further, Japanese PCT Publication No. 505390/1995 proposes a method in which the product stream from an aldolization-dehydration reaction using an aqueous alkali catalyst solution is introduced directly, without oil-water separation, to a distillation column for the next step, and from the top of the column, a heterogeneous azeotrope of water and aldehyde, is recovered and subjected to oil-water separation, so that waste water can be discharged in a form which requires no neutralization treatment.
However, like the above-mentioned Japanese Unexamined Patent Publication No. 28109/1978, the method of Japanese PCT Publication No. 505390/1995 requires an extra distillation column and thus an extra cost for the installation. Further, the aldehyde in the azeotrope distilled from the top of the column in this method is unreacted aldehyde as the feed material for the condensation reaction and has a large solubility to water. Accordingly, an extra post-treatment step for recovering the unreacted aldehyde dissolved in the aqueous layer is required at the time of removing formed water by the oil-water separation, thus leading to a complication of the process. Thus, this method was not satisfactory for an industrial application.